Copolymers of maleic acid monoalkyl esters and vinyl alkyl ethers are known in the art. They are used in the preparation of cosmetics, primarily for hair sprays, where they are included as film-forming components. Known in this connection are the half esters including ethanol and butanol whose general formula is given below, where R.sup.1 =CH.sup.3 and R.sup.2 =methyl or ethyl or propyl: ##STR1##
Several processes for the production of the half esters are disclosed in the literature, with all having in common that the half esters are formed by osterification of maleic anhydride/alkyl vinyl ether copolyemrs, hereinafter called MSA/AVE. Numerous processes are also known for the production of the MSA/AVE copolymers.
Some of these processes employ benzene or toluene as the solvent, for example U.S. Pat. No. 2,782,182. The production of highly molecular maleic anhydride/alkyl vinyl ethers is disclosed in British Patent No. 1,117,515. In this process, methylene chloride is employed as the solvent.
There are some more recent processes which are able to operate without solvents. For example, German Patent No. 3,712,265 discloses a process characterized in that the radical copolymerization is effected in an excess of alkyl vinyl ether, with the alkyl vinyl ether here performing the dual function of solvent and reactant. The copolymers according to this process are highly molecular and are particularly well suited for the production of dental adhesives. However, due to their high viscosity, these copolymers are not suitable as starting material for the production of half esters.
Processes for producing half esters are disclosed, for example, in DE-OS 1,930,009 and British Patent No. 1,233,468. Since, however, it was always a solvent-containing /AVE copolymer which was esterified, the resulting half esters inevitably also contain solvents. As mentioned above, the presence of even the smallest solvent residue is undesirable in cosmetic formulations and is even forbidden by law in some countries.
The production of copolymers of maleic acid monoalkyl esters and alkyl vinyl ethers is disclosed in DE-0S 3,733,158. This process is characterized in that in each polymerization phase the vinyl alkyl ether component is present in excess in the reaction mixture, the radical copolymerization takes place in acetone and the acetone is removed by distillation during or after the ester formation at temperatures up to 70.degree. C. The products produced by this process, which are further processed as alcoholic solutions, still contain small quantities of acetone (column 5, line 23) which may lead to skin irritation in cosmetic formulations and is therefore undesirable.
It has therefore been the continued desire to obtain half esters which are absolutely free of solvents and are low in viscosity, particularly for cosmetic formulations. Attempts have also been made to find a process according to which the viscosity of the half esters could be easily controlled by varying the reaction parameters and which yields products that are also suitable for use as cosmetics. The actuality of this art is emphasized by German Patent Applications DE-OS 3,733,158 and 3,736,996 which were laid open only in 1989, with the latter relating to a process for the production of MA/AVE copolymers.